selling my car - but what ought I say?

pendulum

Just be truthful and say the car runs fine but the engine management light is on. I bought a car on eBay described that way. You don't have to mention the £400 quote you had. Most people will assume it just needs a new O2 sensor which is inexpensive.

pendulum

By the way, do get it code read at another garage to make sure it actually is a £400 repair and not just an O2 sensor

colino

As above, a decent independent doing a scan and if its the downstream O2 sensor, change out of £200.